Log into your Letsignit account and click on your profile on the top right of your screen:

Click on the "My Company " button:

Then click on your current logo :
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Now, you can import a new one from your computer:
Drag it or import it from a file on your computer.

You can crop it if necessary.

Do not forget to click on "Add logo".

Once your logo saved, update your profile on the bottom right of your screen to finish the process.